Referring to FIG. 1, the present invention relates to dual optical fiber communication systems. Generally, a dual optical fiber communication system 50 consists of two dual optical fiber transceivers 52 that exchange signals or data via a pair of optical fibers 54. Each of the two optical fibers 54 transmits data in a different respective direction. The system 50 provides full duplex communications between a pair of host devices 56, which are typically computers but may also be other types of devices, such as network hubs, network switches, computer or microprocessor controlled devices, and computer assisted devices.
Dual optical fiber communication systems, such as the system 50 shown in FIG. 1, are closed systems (i.e., there are no accessible laser emissions) during normal link operating conditions. It is only during maintenance and service conditions when the optical path is accidentally or purposefully broken that access to laser emissions is possible. In other words, if either of the optical fibers 54 is broken or removed from its respective terminal of one of the transceivers 52, laser light may be emitted outside the optical fibers.
The Electrotechnical Commission (IEC) 825 laser standard specifies requirements for a laser product to be classified as a "Safety Class 1" product. This standard defines a maximum allowed level of laser emissions.
The open fiber control protocol (OFC protocol), developed by IBM Corporation, and the subject of U.S. Pat. No. 5,136,410, is a technique for establishing a communication link between two dual fiber communication modules. U.S. Pat. No. 5,136,410 is hereby incorporated by reference as background information. If the two modules are properly interconnected by a pair of optical fibers, communications are established after the two modules exchange a predefined "on-off-on" sequence of pulses. If there are any breaks in the optical communication paths, the laser pulses on the broken path or paths are of sufficiently short duration and average power to meet the strict laser safety standards of a Safety Class 1 product. The repetitive pulse technique used by the OFC protocol reduces the average intensity of laser emissions into the environment in the vicinity of any break in the optical communication path because the laser transmitter is turned on less than 5% of the time whenever an open link condition exists.
The OFC protocol is the subject of ANSI standard X3T11 FC-PH Rev. 4.3.
Standard dual fiber link communication modules, which do not use the OFC protocol, simply turn on their laser transmitter (which is a laser diode) as long as the module is powered on. Standard modules do not attempt to turn off the transmission laser if a complete round trip communication path is not established. However, in most standard module implementations, the host device will not attempt to transmit data on the outgoing optical fiber unless the module detects the receipt of light from the device, if any, on the other end of the dual optical fiber communication channel. Thus, the "standard module protocol" is to always turn on the module's laser transmitter when the module is powered on, but to enable data transmission only when a full duplex connection is established, as indicated by the receipt of light at the module's optical receiver.
While the OFC protocol is an effective technique for meeting the Safety Class 1 product standard, new versions of the standard dual fiber link communication module also meet the Safety Class 1 product standard, even though they continuously transmit laser light when a round trip communication path has not been established. In particular, the new version of the standard module uses very sensitive photo diode receivers, which in turn enables the use of low power laser diode transmitters. Typical laser diode transmission power in such standard modules is approximately 0.25 milliwatt (and the maximum laser transmission power for such devices is approximately 0.4 milliwatt), while typical laser diode transmission power in OFC compliant modules (i.e., communication modules using the OFC protocol) is approximately 0.7 to 1.0 milliwatt. By using laser diode transmitters with such low output power, these standard dual fiber link communication modules meet the Safety Class 1.
The new, low power, standard dual fiber link communication modules are in widespread use, and are incompatible with OFC compliant modules. In other words, an OFC compliant module cannot establish a communication link with a standard module. Given the widespread use of both standard modules and OFC compliant modules, their incompatibility is inconvenient to the vendors of optical fiber communication services, the vendors of optical fiber communication products, as well as to end users who find that new optical fiber communication channels often don't work on the first try due to the use of incompatible communications modules on the two ends of the channel.
It is a goal of the present invention to provide a dual fiber link communication module that is compatible with both OFC compliant modules and standard modules.
Another goal of the present invention is to provide a dual fiber link communication module that meets the Safety Class 1 product standard. That is, the communication module should meet the Safety Class 1 product standard when the other end of the dual optical fiber communication channel is connected to a standard module, an OFC compliant module, another other device, or no device at all.